Delta Sigma (ΔΣ) analog to digital converters are used for high-fidelity conversion of analog signals into digital form. In the context of many signal chains, especially those associated with communications systems, the analog to digital converter forms one of the components in an automatic gain control (AGC) loop. The digital output of the ΔΣ converter is processed by a digital block which controls the gain of the loop such that the various signal processing elements in the loop are operating properly. There may be one or several variable gain elements in the loop including the analog to digital converter itself where its full-scale is adjustable.
Filtering (which often includes decimation) is generally needed in order to properly measure the signal level. This is so because the output of a ΔΣ converter differs markedly from its input on an instant-by-instant basis due to the large amount of quantization noise that is present in the converter's output. This large discrepancy corrupts the signal-strength estimation process, necessitating the use of filtering.
However, filtering has its own drawbacks. One problem with filtering is that it removes both quantization noise and interferer signals. If the interferer signals dominate the signal strength, then the signal-strength estimate will be in error. Thus, using a filter that is narrow enough to eliminate all out-of-band noise (and hence all out-of-band signals) can result in a signal-strength estimate which is too low. This low estimate will cause the gain of the variable gain element to be set too high, possibly so high that the out-of-band signals overload the analog to digital converter. Another problem with filtering is that it takes time and thus limits the speed of the automatic gain control loop.